IP Multimedia Subsystem (IMS) is a subsystem for providing Internet Protocol (IP) multimedia service put forward by 3rd Generation Partnership Project (3GPP). In the IMS, a Packet Switch (PS) domain is employed as a channel for bearing upper layer control signaling and media transfer, and Session Initial Protocol (SIP) is introduced as service control protocol. Service control and bearer control are separated, so as to provide abundant multimedia services.
The main functional entities in the architecture of IMS network include a Call Session Control Function (CSCF) entity for controlling functions such as subscriber registration and session, a Home Subscriber Server (HSS) for collectively managing the subscription data of a subscriber, and an Application Server (AS) for providing various service logic control functions. In addition, the architecture of IMS network further includes a Multimedia Resource Control Function (MRCF) entity and a Policy Decision Function (PDF) entity, etc. And, in terms of the roles and functions, the CSCF may be further divided into Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF), serving-CSCF (S-CSCF) etc., which logically accomplish different functions in SIP session routing respectively, and physically may be set integrally or separately. A proxy node P-CSCF in a location where a subscriber presently lies is used by the subscriber to access the IMS, a home domain service node S-CSCF in a location where the registration is made is used to accomplish the session and service trigger control and the service control interaction with the AS, and the I-CSCF performs the function of routing query.
In view of the architecture, the IMS is a network irrelevant to access technologies. Subscribers may experience the same service no matter whether Asymmetric Data Subscriber Line (ADSL) computer access or Wideband Code Division Multiple Access (WCDMA) mobile phone wireless access is used. The architecture of IMS network is divided into an access interconnection layer, a session layer and an application layer.
The access interconnection layer mainly accomplishes the functions of initiating and terminating an SIP session of various SIP terminals, implementing conversions among various bearer types of IP packet bearers, implementing various policies of Quality of Service (QoS) according to deployment of service and control of the session layer, and accomplishing the interconnecting and interworking with a traditional Public Service Telecommunication Network (PSTN)/Public Land Mobile Network (PLMN). The access interconnection layer includes various SIP terminals, wired access, wireless access, interconnecting and interworking gateway, etc.
The session layer accomplishes basic session control, accomplishes subscriber registration and SIP session routing control, interacts with the application server to perform functions of session, subscriber data maintenance and management, service QoS policy management, etc., and provides a consistent service environment for all the subscribers in conjunction with the application layer. When networking is practiced, for the division and deployment thereof, it is necessary to take comprehensive consideration of the access mode of IMS service, the location of the IMS access point, the capacity and capability of the CSCF, and demand on traffic of the subscribers. In addition, the topology hiding and interworking demand of the operator network are also considered. The P-CSCF is an ingress through which a User Equipment (UE) accesses the IMS system, so that the functions of Proxy and UserAgent in SIP protocol are implemented. The S-CSCF plays a key role in an IMS core network. The S-CSCF is responsible for UE registration authentication and session control, performs basic session routing function for IMS subscribers of a calling terminal as well as a called terminal, and performs value-added service trigger and service control interaction to the AS when conditions are met according to IMS trigger rules subscribed by the subscriber. The I-CSCF functions as a gateway node in the IMS core network. The I-CSCF provides functions of service node allocation for the subscriber in the current domain, routing query, and topology hiding between different IMS, and determines which S-CSCF provides services to the subscriber according to various combined conditions.
The application layer provides the subscriber with service logic, including implementing traditional basic telephone services, such as call transfer, call wait, conference, etc. The IMS implements inheritance of intelligent service existing in a Circuit Switch (CS) and a Packet Switch (PS) by use of interworking of an IP Multimedia-Services Switching Function (IM-SSF) entity with traditional intelligent service.
In addition to existing service in the CS and the PS, the IMS architecture may also provide SIP-based nontraditional telecommunication service via the AS, such as multimedia service, talk-back mobile phone, Presence, etc. Additionally, the IMS may provide a simple Application Program Interface (API) via an Open Services Access-Gateway (OSA-GW), so that a third party may securely use network resources and provide service via the interface, implementing abundant entertainment and game services. In addition, the IMS fully considers the demand of practical operation and works out specifications on QoS, security, billing, and interworking with other networks.
The SIP is a basic protocol of an IMS control layer, is one of multimedia communication system frame protocols, is an application layer protocol for establishing, changing or terminating a multimedia session, and accomplishes session establishment and media negotiation in the IMS in cooperation with a multimedia stream protocol. As a control layer protocol, the advantage of the SIP lies in that it is based on an open Internet standard, readily implements interconnecting and interworking among different networks as well as more abundant service features, supports a function of mobility of the application layer, and has simplicity and well-known extension potential.
SIP messages are divided into a Request message which is sent from a client to a server and a Response message which is sent from the server to the client. An SIP message consists of a Start-Line, a message header consisting of one or more Fields, a CRLF marking the end of the message header, and an optional Message Body. Message headers are divided into a General-Header, a Request-Header, a Response-Header and an Entity-Header. Here, a header that describes the message body is referred to as an Entity-Header. Start-Lines are divided into a Request-Line and a Status-Line. Here, the Request-Line is the Start-Line of a request message, and the Status-Line is the Start-Line of a response message.
The SIP implements control of a call mainly by use of the following six methods: INVITE, which indicates that a subscriber or service joins a session, and the message body part of which contains information description of the called; ACK, which is mainly adapted to acknowledge that a client terminal has responded to the request of the INVITE; BYE, which is adapted to send by the client a message to the server to terminate the call; CANCEL, which is adapted to cancel a suspended call; REGISTER, which is adapted to register information on the client with a location server; and OPTIONS, which is adapted to query information on and functions of the server. The SIP mainly defines the following five types of response statuses: 1xx: information, which indicates that the request has been received and the request may be processed subsequently; 2xx: correct, which indicates that the call has been received and processed correctly; 3xx: redirection, which indicates that the call needs to be redirected and processed; 4xx: client error, which indicates that a representation error exists in the message and the message can not be processed by the server; and 5xx: server error, which indicates that the server can not process the message.
In the SIP model, in order to establish a session, a subscriber proxy client terminal initiates a request to a subscriber proxy server for routing in the network via the subscriber proxy server. In addition, a registration server provides location information of the subscriber proxy client terminal, because it needs to map the SIP address to an IP address.
The subscriber proxy client terminal in the IMS is a UE. The subscriber proxy server and registration server in the IMS refer to the CSCFs. Here, the S-CSCF acts as the registration server and activates application service control based on subscriber data; the P-CSCF is the first contact point of the UE in the IMS network, and a SIP signaling message is delivered between the P-CSCF and the UE; and the I-CSCF is the first contact point for an external network, especially for an external IMS network. The IMS uses “home control”, i.e. session control signaling is always in the charge of an S-CSCF that lies in the home network. The P-CSCF may be in the home network or in a visited network.
The IMS is regarded as an objective network of the telecommunication core network. However, a phase of history has to be involved in the evolution from the 3GPP to the IMS.
What Combining CS and IMS services (CSI) concern is how to combine CS domain service of 3G with IMS service.
The function of a Network Domain Selection (NeDS) entity is to determine whether an incoming call from a subscriber terminal is terminated in a CS domain or in an IMS according to related information upon receiving the call. The information dependent upon which the NeDS makes the determination includes registration status information of the subscriber and the terminal in the CS domain and IMS, capability information of an IP Connectivity Access Network (IP CAN) in which the subscriber and the terminal exist, and preference information of the subscriber and the operator.
The IMS also bears Voice over IP (VoIP) service. Therefore, interworking between a VoIP terminal and a CSI capable terminal has to be addressed. When a CSI UE interworks with a VoIP UE, a problem will arise. If the VoIP calls the CSI UE, the VoIP UE establishes a voice session only using an IMS signaling. The IMS session directly reaches an IMS part of the CSI UE. After the CSI UE receives the request message, it directly establishes a VoIP voice session between the two terminals. Due to limitation of the QoS of VoIP on the CSI UE side, the CSI UE can not establish a real-time session connection using the PS domain. For example, the PS domain on the CSI side may not have the capability to provide real-time voice communication service, and thus the interworking between the CSI and the VoIP terminals can not be implemented. Therefore, in the prior art, after the IMS network is converged, complete interworking between the CSI and the VoIP terminals can not be addressed.
In practical applications, the above solution can not address interworking between IMS terminals such as a CSI capable terminal and a VoIP terminal.
The main reason that causes such a case lies in that when a VoIP terminal or other IMS terminal calls a CSI capable terminal, only an IMS PS connection can be established, and the CSI capable terminal can not select a CS domain to bear real-time service.